Molecular sieves are a class of important materials used in the chemical industry for processes such as gas stream purification and hydrocarbon conversion processes. Molecular sieves are porous solids having interconnected pores of different sizes. Molecular sieves typically have a one-, two- or three-dimensional crystalline pore structure having pores of one or more molecular dimensions that selectively adsorb molecules that can enter the pores, and exclude those molecules that are too large. The pore size, pore shape, interstitial spacing or channels, composition, crystal morphology and structure are a few characteristics of molecular sieves that determine their use in various hydrocarbon adsorption and conversion processes.
For the petroleum and petrochemical industries, the most commercially useful molecular sieves are known as zeolites. A zeolite is an aluminosilicate having an open framework structure formed from corner-sharing the oxygen atoms of [SiO4] and [AlO4] tetrahedra. Mobile extra framework cations reside in the pores for balancing charges along the zeolite framework. These charges are a result of substitution of a tetrahedral framework cation (e.g., Si4+) with a trivalent or pentavalent cation. Extra framework cations counter-balance these charges preserving the electroneutrality of the framework, and these cations are exchangeable with other cations and/or protons.
Synthetic molecular sieves, particularly zeolites, are typically synthesized by mixing sources of alumina and silica in an aqueous media, often in the presence of an organic structure directing agent or templating agent. The structure of the molecular sieve formed is determined in part by the solubility of the various sources, the silica-to-alumina ratio, the nature of the cation, the synthesis conditions (temperature, pressure, mixing agitation), the order of addition, the type of structure directing agent, and the like.
Although many different crystalline molecular sieves have been discovered, there is a continuing need for new molecular sieves with desirable properties for gas separation and drying, hydrocarbon and chemical conversions, and other applications. New molecular sieves may contain novel internal pore architectures, providing enhanced selectivities in these processes.